1. Field of the Invention
This invention relates to a dust core material mainly composed of soft magnetic powder, such as iron powder or iron-based alloy powder, and phenolic resin fine powder, a dust core obtained from such a material as mentioned above, and a method for making the same. The dust core obtained according to the invention exhibits excellent mechanical strength and magnetic properties at room temperatures and also at high temperatures.
2. Description of Related Art
In the core employed within an AC magnetic field, it is necessary that not only low iron loss, particularly low eddy current loss, and a magnetic flux density be high, but also no breakage take place in the course of handling in a manufacturing process and winding for making a coil. With a so-called dust core, the eddy current loss can be suppressed by causing an insulating resin to intervene between iron particles, and the resin serves as a bonding agent for the iron particles, so that it becomes possible to prevent the breakage while ensuring good mechanical strength.
It is conventionally known that dust cores are obtained by subjecting, to compaction molding in a desired form, mixtures of soft magnetic powder such as iron powder and organic binder resins such as epoxy resins, polyimide resins, silicone resins, phenolic resins, nylons or the like. Moreover, the dust core has been mass-produced in such a way that in order to reduce the mutual frictional resistance of powdery particles and frictional resistance with a die during the course of compaction molding, a lubricant, such as zinc stearate or lithium stearate, is mixed in an amount of approximately 0.8 to 1 wt % (e.g. Japanese Laid-open Patent Application Nos. Sho 56-74902 and Sho 62-232102, Japanese Patent Publication Nos. Sho 58-46044 and Hei 4-12605, Japanese laid-open Patent Application No. Hei 11-195520, and Japanese Patent Application No. 2000-49008).
Since a dust core is conventionally employed at room temperatures and has not been applied to parts requiring mechanical strength, mechanical strength, especially, at high temperatures has been aside from the question. Although known dust cores using such a resin as mentioned above have large mechanical strength at room temperatures, the mechanical strength lowers at a temperature as high as 100xc2x0 C. or over owing to the glass transition or softening of the resin. When using a nylon resin among those resins indicated above, mechanical strength at a high temperature significantly lowers. This tendency is true of hot cured resins such as epoxy resins, polyimide resins, phenolic resins and the like. Thus, where the core is used at high temperatures or becomes high in temperature due to the generation of heat in use, it has been difficult to apply to parts that require satisfactory mechanical strength.
For a technique of improving mechanical strength of dust core, it has been proposed to use a lubricant mixed with starting materials, which has a melting point higher than a curing temperature of a curable binder resin (Japanese Patent Publication No. Hei 4-12605). However, because the real strength of the dust core is determined depending on the bonding force or adhesion force of binder resin, such a technique as mentioned above wherein a lubricant that impedes the bonding between iron powder and resin is merely excluded only in the course of curing the binder resin has been unsatisfactory for improving mechanical strength at high temperatures.
Besides, for a measure of improving the a green density, there is proposed a technique wherein a lubricant is applied onto the inner wall surfaces of a die without addition of a lubricant in a starting mixed powder (Japanese laid-open Patent Application No. Hei 9-272901). Since the lubricant acts to impede the bonding between the iron powder (soft magnetic powder) and the resin thereby causing mechanical strength to be lowered, it can be expected that this technique is effective for improving not only the green density, but also the green strength. However, in order to improve mechanical strength at high temperatures, it is necessary to improve the mechanical strength of a binder resin per se as stated hereinabove.
Further, it is required to impart satisfactory electric insulating properties so as to suppress the eddy current loss. From this point of view, it is necessary to uniformly mix a binder resin and a soft magnetic powder prior to compaction. The uniformity of such a mixture of soft magnetic powder/binder resin is important from the standpoint of improving the mechanical strength of the dust core obtained by compacting the mixture. In this connection, however, a phenolic resin is, for example, in the form of a liquid, a mass or flakes and has to be mixed with soft magnetic powder after dissolution in a hydrocarbon solvent such as toluene, xylene hexane or the like, with a difficulty in working properties.
An object of the invention is to provide a mixed powder which is a uniform mixture of a soft magnetic powder and a binder resin, has such an electric resistance that an eddy current mutually occurring in-between soft magnetic powder particles can be suppressed, and has high mechanical strength, thus being suited as a starting material for dust core.
Another object of the invention is to provide a dust core obtained from such a mixed powder as mentioned above.
A further object of the invention is to provide a method for making such a dust core.
The above objects can be achieved, according to the invention, by a powder for high strength dust core (which may be sometimes referred to merely as xe2x80x9cpowder for dust corexe2x80x9d) which comprises a soft magnetic powder and a powder of a phenolic resin wherein when 1 g of the phenolic resin is dissolved in 100 ml of boiling methanol, an undissolved matter is left in an amount of at least 4 wt % based on the total amount of the phenolic resin.
The phenolic resin fine powder should preferably have an average particle size of 30 xcexcm or below. The use of the resin powder having such an average particle size ensures more uniform mixing with a soft magnetic powder.
The content of the phenolic resin fine powder in the mixed powder for dust core should preferably range from 0.5 to 5 wt %. It is recommended that the mixed powder further comprises at least 0.2 wt % of a lubricant. It will be noted that in case where the powder for dust core is used for a compaction process using a die that is applied with a lubricant on the inner wall surfaces thereof, the amount of the lubricant should preferably be not larger than 0.2 wt % (inclusive of 0 wt %).
The high strength dust core (which may be sometimes referred to merely as xe2x80x9cdust corexe2x80x9d) of the invention is one that is obtained by curing the phenolic resin present in a compaction product of the powder for dust core. More particularly, the method for making a high strength dust core according to the invention comprises the steps of compacting the above-defined powder for dust core and curing the phenolic resin in the resulting compaction product.